Memory cells are commonly formed in integrated circuits by energy storage devices, such as capacitors. A capacitor is composed of two layers of a material that is electrically conductive (hereinafter, electrodes) brought near to one another and separated by a material that is electrically nonconductive. The capacitor is connected to an energy source (e.g. battery) with a certain voltage level (hereinafter, energy level). Charge flows from the energy source to the capacitor until the capacitor exhibits the energy level of the energy source. Then, the capacitor is disconnected from the energy source. The capacitor will exhibit the energy level of the energy source until the charges stored in the capacitor are removed either by design or unintentionally.
This ability of the capacitor to “remember” an energy level is valuable to the operation of semiconductor integrated circuits. Often, the operation of such circuits may require that data be stored and retrieved as desired. Because of its ability to remember, the capacitor is a major component of a semiconductor memory cell. One memory cell typically stores one bit of data. A system of memory cells is a semiconductor memory array where information can be randomly stored and retrieved from each memory cell. Such a system is also known as a random-access memory.
One type of random-access memory is dynamic random-access memory (DRAM). The charges stored in DRAM tend to unintentionally leak away over a short time. It is thus necessary to periodically refresh the charges stored in the DRAM. Even with the refresh burden, DRAM is a popular type of memory because it can occupy a very small space on a semiconductor surface. This is desirable because of the need to maximize storage capacity, i.e. the number of stored bits, on the limited surface area of an integrated circuit.
Examples of some refresh schemes are disclosed in U.S. Pat. No. 6,097,658 issued to Satoh et al.; U.S. Pat. No. 6,094,705 issued to Song; and U.S. Pat. No. 4,631,701 issued to Kappeler et al.; all incorporated herein by reference.
Many modern electronic devices are portable devices which use a battery as its energy source. As more and more portable electronic devices are developed and find commercial acceptance, it is desirable to conserve energy. Conservation of energy is driven by the need to use smaller and lighter power supplies so the electronic devices incorporating the power supply are themselves smaller and lighter. Moreover, there is a need to conserve energy so that the energy source lasts longer and the electronic device can operate longer between energy source charges or replacement. Accordingly, power consumption in memory devices used in electronic devices is becoming increasingly important.